I suggest SpamAssassin (http://www.spamassassin.org)
It's the best out there, and, yes, it's free :-)
There's an extensive doc on installing it on different MTAs (including sendmail) :
http://wiki.apache.org/spamassassin/IntegratedInMta

(using the spamd daemon based version of procmail rather than the standalone app - probably recommended for 100 users)

Remember that spamassassin marks the mail as SPAM (adds headers and changes the message if it is over the global spam threshold) - your users may want to control their filtering within their own mail client or .procmailrc by filering for the expression in the headers:

X-Spam-Level: *****

(with the appropriate number of *'s - less is stricter)

Recent versions can also "learn" on a per user basis using Bayesian filtering, but it requires each user to have shell access (or a cron script to scan the spam and ham folders) and so is a bit more messy.

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